Amplifier coupling circuit



Jan. 20, 1942. D. E. NORGAARD 2,270,764

AMPLIFIER COUPLING CIRCUIT Filed April 5, 1940 7 Fig.2.

ourPur mm m/Pur FREQUENCY Inventor":

- Donald E.Nor aard, by Wan/1 7 His Attorney.

Patented Jan. 20, 1942 AMPLIFIER COUPLING CIRCUIT Donald E. Norgaard,Cohoes, N. Y., assignor to General Electric Company, a corporation ofNew York Application April 5, 1940, Serial No. 328,045

5 Claims.

This invention relates to an amplifier coupling circuit and moreparticularly to a coupling circuit for use in a wide frequency bandamplifier.

It is an object of my invention to provide an improved wide bandcoupling device in which phase shift and variations in amplituderesponse at low frequencies are substantially eliminated withoutaltering in any way the characteristics, such as high amplification andconstant phase shift, at high frequencies.

-It is another object of my invention to provide such an improvedcoupling device in which the ratio of output voltage to theinput currentand the time delay therebetween are substantially constant for alltransmitted frequencies and in which a large amount of amplification isattained.

It is a further object of my invention to provide a wide band couplingdevice, as set forth above, which is substantially independent of powersupply impedance and which reduces the effect of interfering powersupply ripple.

It is as well an object of my invention to provide a wide band couplingcircuit which is easily applied in the mechanical design of anamplifier, and which can be adjusted by simple methods to givesubstantially uniform response over the desired range of frequencies.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. 1 illustrates a circuit embodying myinvention; Fig. 2 illustrates a simplified circuit in which certaincharacteristics are similar to those of the circuit of Fig. 1; and Fig.3 represents certain operating characteristics of the circuit of Fig. 1both before and after the application of my invention.

Referring to Fig. 1, the amplifier illustrated therein comprises asource II] of a Wave whose frequency lies in a very wide band, and apair of electron discharge amplifying devices II and I2, which areconnected in cascade to supply the amplified wave to a load device I3.The source I is connected in series with a source I4 of grid biaspotential between the control grid I5 and the cathode I6 of the deviceII. The anode I! of the device II is connected through an inductance I8,a resistance I9, and a resistance in series to the positive terminal ofa source of potential 2 I. An intermediate point of the source 2| isconnected to ground and to the cathode It.

A by-pass condenser 22 is connected from ground to a point between theresistors l9 and 20, so that high frequency currents from the anode I'Iflow through the inductance I8, resistance I9 and condenser 22. Thiscondenser is sufficiently large that little high frequency voltage isproduced therein. A screen electrode 23 of the device II is maintainedat a substantially constant positive potential through a resistor 24connected between the electrode 23 and the positive terminal of thesource 2|. A high frequency by-passing condenser 25 and a gas dischargeregulating device 26 are connected in shunt from the electrode 23 to thecathode I 6 to aid in maintaining the potential of the electrode 23 morenearly constant at both high and low frequencies.

High frequency voltage variations appearing on the inductance I8 and theresistance I9 are transmitted to the discharge device I2 through a smallcoupling condenser 21 which is connected between the anode I1 andcontrol grid 28 of the device I2. A grid resistor 29 is connected fromthe grid 28 to the negative terminal of the source 2| which maintainsthe grid at a suitable negative bias potential. The negative terminal ofthe source 2| is by-passed to ground and to the cathode 3B of the deviceI2 by a large by-passing condenser 3|. The load device I3 is connectedbetween anode 32 of the device I2 and the positive terminal of thesource 2|. Screen electrode 33 of the device I2 is connected throughresistor 34 to the positive terminal of the source 2| and is by-passedto ground and to the cathode by a by-passing condenser 35 and a gasdischarge voltage regulating device 36 in shunt, which maintain thepotential of the screen electrode 33 substantially constant.

The wave from the source I0 is amplified and transmitted to the loaddevice I3 as follows: Voltage variations produced by the source I0 onthe control grid I5 excite corresponding current variations in the anodeI! through the anode load circuit comprising the inductance I8, theresistance I9, and the shunt combination of resistance- 20 and thecondenser 22. The discharge device II possesses an extremely highimpedance, sothat voltage variations of the anode I1 caused by currentflow in the anode load circuit have substantially no effect on the anodecurrent. These voltage variations are transmitted through the couplingnetwork comprising condenser 27, the condenser 38, and resistors 31 and39 to the control grid 28 of the device I2 where they are againamplified and appear as large variations in current in the anode 32 andthe load device l3.

The coupling condenser 2! is made small in order to minimize its straycapacity to ground. It is therefore incapable of transmitting faithfullylow frequency components of the wave from the source H1. The means whichare provided in accordance with my invention, in shunt with the couplingcondenser 21 for transmitting these low frequency components, comprises,in order, a series connection of the resistor 31, the large couplingcondenser 38, and the resistor 39. The

- components of the entire coupling circuit described are designed, aswill be explained later, to transmit all frequencies within anexceedingly wide range with high gain and at the same time withsubstantially uniform response.

Fig. 2 illustrates the circuit of Fig. 1, simplifled to produce idealoperation with constant phase shift and a constant ratio of inputcurrent to output voltage at any frequency, if it be assumed that straycapacity to ground is negligible. In practice this simplified circuitresponds ideally only at low frequencies, because stray capacity isinevitably present. For the lower portion of the frequency band to be'transmitted the circuit of Fig. 2 is therefore equivalent to that ofFig. 1, with certain reservations to be made clear as the explanationproceeds.

It will be noted that the condenser 38, which separates the resistors 39and 3! of Fig. 1 is not shown in thisfigure. For purposes of thisexplanation it is assumed that this condenser is infinitely large, Thisintroduces no error except at the very lowest frequencies, which are notintended to be transmitted. It has been found that the circuit of Fig. 2allows a wave covering a frequency range occupying the lower part of thedesired band to be transmitted therethrough with uniform voltageresponse and substantially zero phase angle, provided theresistance-capacity products of the left and .right portions of thecircuit be made equal. In other words, the condenser 21 and resistors3'! and 39 produce a phase shift which varies with frequency, but thecondenser 22 and resistors 19, and 29 produce a complementary phaseshift, so thatthe overall phase shift through the circuit of Fig. 2 issubstantially proportional to frequency. That is, if the ratio -ofresistance 29 to resistance I9 be equal to the ratio of capacity 22 tocapacity 21, and also be equal to the ratio of resistance 31, 39 toresistance 20, any current made to flow between input conductors 64 and45 must divide between the lefthand and the right-hand branches in acertain fixed proportion regardless of frequency, since their impedanceshave been made proportional.

In practice, the inclusion of the condenser 38, Fig. 1, between theresistance 31 and 39 introduces some error at very low frequencies,since the combination of these three elements is not purely resistive.However, the condenser 38 can be chosen large enough with respect to theresistors 3? and 39 so that the phase angle of the combination can bemade so small at any specified frequency that the error introduced byuse of condenser 38 can be neglected. It is desired to use the condenser38 in order to prevent amplification of any direct current component ofthe source ID, of Fig, 1.

The amount of amplification actually available in the circuit of Fig. 2is directly proportional to the magnitude of the resistor I9,.this

resistor being given a value in ohms determined by the function 2 1r f 6Where f is the maximum frequency which it is desired to transmit withuniform response and c is the total stray capacity in farads to groundin the coupling stage. The stray capacity includes only the combinedcapacity effective to ground from the anode H, the condenser 21, and

grid 28, and from all intermediate wiring. The

inductance I8 is given a value in henrys which is times the value of theresistance l9, which inductance compensates substantially for thepresence of the stray capacity 0. It is therefore apparent that it isvery desirable to minimize the stray capacity in the coupling circuitsince the value of the resistance l9 may thereby be made a maximum witha corresponding maximum gain of the stage for a given maximum frequency,f.

The choice of resistor 29 must be made with due regard to the maximumdirect current impedance which should be utilized in the control gridcircuit of the device I2. The condenser 22 is made sufficiently large toact effectively as a decoupling filter in conjunction with resistor 20,which is given as high a value as will permit sufficient continuousvoltage to be applied to the anode ll of device II to insure normaloperation. The values of the inductance l8, the resistances I9, 29, 20,and 37, 39, and the condensers 22 and 2'! are thus determined throughuse of the proportionalities pointed out above.

As stated above, it is highly desirable to minimize the stray capacityto ground from the coupling circuit and the components of the couplingcircuit are fixed in size with this end in view. As" indicated by dottedlines in Fig. 1, which represent the case or shell of the condenser 38,and by a dotted capacitance 40, the condenser 38 may have a substantialamount of stray capacity to ground because of the possibility that itwill have large physical dimensions. The dotted capacitance 40 isintended to represent the effective shunt capacity from the capacitor 38to ground, whether the case be grounded or not. If the resistors 31 or39- are not made sufiiciently large and resistor 20 is omitted, thisstray capacity affects the coupling circuit so as to reduce theamplification therethrough at high frequencies. This tendency isillustrated by curve 4| in Fig. 3 in which values of the ratio of theoutput voltage applied to grid 28 of device l2 to the input voltageapplied to grid I5 of device H for such a coupling circuit are plottedas ordinates against values of frequency as abscissae. This curve showsclearly that if the stray capacity 40 be allowed to affect such acircuit, otherwise properly adjusted, the intermediate frequencies aretransmitted in too large amount and the highrfrequencies are transmittedin too small amount.

In my invention, the ratio which is established between resistors 29 andI9 must be preserved between resistors 37, 39 and resistor 20 inaccordance with the previous explanation concerning Fig. 2 so that thelow frequency response of the coupling circuit is made highly uniform.It is thus necessary to choose both the ratio between resistors 20 and'I9 and the value of resistor 20 high enough to prevent loss ofhighfrequen'cy response through the shunting effect of the networkcomprising resistors 31, 39 in series with condenser 40 to ground. Thisnetwork would otherwise have sufiiciently low impedance with respect toresistor I9 to by-pass a relatively large amount of high frequencycurrent.

If the resistors 31 and 39 be made too small as a result of too low avalue for resistor 20 or too low a ratio between resistors 29'and I9, orif, for example, resistor 39 be omitted (resistor 31 being thenincreased to maintain uniform low frequency response as previouslyshown) and the remaining components in the circuit be properly adjustedin accordance with the above explanation, taking into account the straycapacity 40 which is thus added to the existing stray capacity, cmentioned above, the response of the coupling circuit is madesubstantially linear for all frequencies but does not permit a maximumamount of amplification. This fact follows from the necessity ofreducing the value of resistor I9 to take account of increased effectiveshunt capacity when the amplifier is made to pass a given maximumfrequency f with uniform response. The curve 42 of Fig. 3 illustratesthe frequency response of the circuit of Fig. 1 when it is adjusted insuch a fashion.

In accordance with my invention, it is best practice to make theresistors 31 and 39 of roughly equal value and sufficiently large toserve as decoupling resistors for high frequency currents. By makingthese resistors of such a size,

the effect of the stray capacity may be made negligible, so that, if theremaining components of the circuit be proportioned properly (takinginto account all stray capacity except that represented by the capacity40), a maximum amount of amplification is obtained. The characteristicsof a circuit constructed in such a manner are represented by the curve43 of Fig. 3.

Any inherent internal impedance of the power source 2I in Fig. 1 iseffectively in series with the resistor 20, which is made as large aspossible without dropping the direct current plate potential of device II to such a value that its operation is impaired. Thus, the internalimpedance of the source 2| is normally small compared with the resistor20, and its effect on the operation of the circuit is negligible.Likewise, any low frequency ripple appearing in the source 2I must passthrough the high resistance 20 to the bypass condenser 22, throughresistor I9 and inductance I8 in order to appear on the anode I! of thedevice II. Moreover, the network consisting of condenser 21, resistor31, condenser 38, and resistor 39 operating in conjunction with resistor29 serves further to attenuate low frequencies which may appear in thesource 2|. The amount of such attenuation approaches as a limit thequotient of resistor 20 plus resistor I9 divided by resistor I9. Thecombination of the resistor 20 and the condenser 22 serves as a resistance-capacity filter which itself reduces the amount of ripple thatappears on the anode I I of device II at the outset. This same filteraction takes place at high frequencies where the attenuation of theright hand branch of the coupling network is small, but the attenuationof the combination of resistor 20 and condenser 22 is correspondinglygreater. Since the anode circuit is made up of elements whose impedanceis independent of current, voltage, or frequency, high frequencycurrents from the anode I! of device I I are prevented from reaching thepower source 2| in any great amount.

Since it is possible to neglect the stray capacity 40 of Fig. l by theapplication of my invention, the mechanical design of the wide bandamplifier is greatly simplified, both as to placement of the circuitelements and as to the necessity of shielding to prevent unwanted straycouplings in a multiple-stage amplifier. The only require ments of thecondenser 38, for instance, are that its stray capacity to case orground be small with respect to its nominal capacity and that itsnominal capacity be large enough to make the series circuit consistingof resistors 3'! and 39 and condenser 38 essentially resistive over thelow frequency portion of the frequency band it is desired to transmit.Because condenser 38 is decoupled from the signal circuit at highfrequencies by resistors 31 and 39 there is no need to isolate it orshield it from other parts of the amplifier; moreover, its nominalcapacity can be made large, so that the low frequency response can beextended downward to practically any point. Since the value of thecoupling condenser 21 can be made small by proper choice of the otherconstants of the circuit, its stray capacity to ground and to otherparts of the circuit can be made a minimum with the result that highergain or wider bands of amplification can be had than with other couplingcircuits which have equivalent low frequency response.

In a typical example in which the discharge devices II and I2 are of theRCA type 1852, the various components of the circuit may be given thefollowing values:

Resistor l9 2000 ohms (depends on stray capacity) Inductance l8 0.05millihenry (depends on stray capacity) Capacity 27 0.02 microfaradResistor 29 1.0 megohm Condenser 22 10.0 microfaraols Resistor 37 2.5megohms Resistor 39 2.5 megohms Resistor 20 10,000 ohms Condenser 38 1.0microfarad Source 21 350 volts positive with respect to ground, and 2.0volts negative with respect to ground Devices 26 and 36 volts A couplingcircuit having components with these values gives substantially constantresponse (see Fig. 3, curve 43) from 20 cycles to 3 megacycles with a 20cycle phase shift substantially less than 0.1 degree. If it be desiredto make the low frequency transmission and phase shift even better, thecondenser 38 may be increased in magnitude to 2 microfarads. The 20cycle phase shift is then substantially less than 0.05 degree, and thedecoupling provided by the resistors 3'! and 30 for the stray capacityis still ample, so that the high frequency characteristic is notmeasurably altered.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto, sincedifferent modifications may be made both in the circuit arrangement andinstrumentalities employed without departing from the principledisclosed above, and I aim by the appended claims to ,cover any suchmodifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Coupling means for transmitting a wide frequency band wave betweentwo terminals, comprising a first resistor connected between one of saidterminals and a fixed potential point, a second and a third resistorconnected between the other of said terminals and a fixed potentialpoint, a point between said second and third resistors being connectedto a fixed potential point through a by-pass capacitor in shunt to saidthird resistor, a small capacitor and a large capacitor connected in ashunt circuit around said small capacitor to provide respectively highfrequency and low frequency coupling between said terminals, said largecapacitor having substantial stray capacity tending to reducetransmission of high frequency components between said terminals, and adecoupling resistor in said shunt circuit on each side of said largecapacitor providing sufficient decoupling between said stray capacityand said small capacitor to maintain transmission of said high frequencycomponents substantially unaffected, the ratios of said first resistorto said second resistor, of the sum of said decoupling resistors to saidthird resistor, and of said by-pass capacitor to said small capacitorbeing substantially equal.

2. Coupling means for transmitting a wide frequency band Wave betweentwo terminals, comprising a first resistor connected between one of saidterminals and a fixed-potential point, a series connected groupincluding an inductance, a second resistor and a third resistorconnecting the other of said terminals to a fixed potential point, apoint between said second and third resistors being connected to a fixedpotential point through a by-pass capacitor in shunt to said thirdresistor, a small capacitor and a large capacitor connected in a shuntcircuit around said small capacitor to provide respectively highfrequency and low frequency coupling between said terminals, saidcoupling means including said large capacitor having "substantial straycapacity tending to reduce transmission of high frequency componentsbetween said terminals and said inductance tending to compensatetherefor, and a decoupling resistor in said shunt circuit on each sideof said large capacitor providing sufficient decoupling between saidstray capacity and said small capacitor to maintain transmission of saidhigh frequency components substantially unaffected, the ratios of saidfirst resistor to said second resistor, of the sum of said decouplingresistors to said third resistor, and of said by-pass capacitor to saidsmall capacitor being substantially equal.

3. Coupling means for transmitting a wave having components within apredetermined band of frequencies, comprising a small condenser and alarge condenser connected in a shunt circuit around said smallcondenser, said large condenser having substantial stray capacitytending to reduce transmission of high frequency components within saidhand through said condensers, decoupling means in said shunt circuit oneach side of said large condenser for decoupling said stray capacityfrom said small condenser at the frequencies of said high frequencycomponents so that such components remain substantially unaffected, saidcoupling means being effective to transmit a wave having componentswithin said band of frequencies through said condensers so that thephase shift of said waves passing through said coupling means is notproportional to frequency, and means for producing a phase shift in saidwave complementary to the phase shift of said wave through said couplingmeans so that the overall phase shift through said coupling means andsaid last means is substantially proportional to frequency.

4. Coupling means for transferring a signal represented by the intensityof a current from an input terminal to an output terminal on which saidsignal is represented by a voltage, said signal having frequencycomponents within a predetermined band of frequencies, and saidterminals being at different continuous potentials, said coupling meanscomprising a small condenser connected between said terminals, a largecondenser connected in a shunt circuit around said small condenser, saidlarge condenser having substantial stray capacity tending to reducetransmission of high frequency components through said condensers, anddecoupling means in said shunt circuit on each side of said largecondenser for decoupling said stray capacity from said small condenserat high frequencies within said predetermined band to maintaintransmission of said signal substantially unaffected by said straycapacity, said coupling means being effective to transmit said signalthrough said condensers so that the phase shift between variations ofsaid input current and corresponding variations of said output voltageis not proportional to frequency, and means for producing an additionalphase shift in said output voltage complementary to said first phaseshift through said coupling means so that the overall phase shiftbetween said input current and said output voltage is substantiallyproportional to frequency.

5. Coupling means for transmitting waves having components within apredetermined band of frequencies, comprising a small condenser, a largecondenser connected in a shunt circuit around said small condenser, saidlarge condenser having substantial stray capacity tending to reducetransmission of waves of high frequencies within said band through saidcoupling means, decoupling means in said shunt circuit on each side ofsaid large condenser to reduce the effective stray capacity of saidcoupling means for said high frequency waves to a relatively smallerresidual stray capacity, means for broadly tuning said residual straycapacity at such high frequencies thereby to achieve a large voltageresponse through said coupling means at such frequencies, said couplingmeans having such characteristics that the phase shift of low frequencywaves within said band passing through said coupling means is notproportional to frequency, and a compensatory phase shifting network incircuit with said coupling means, said network including impedanceelements which are proportioned to produce a phase shift in said lowfrequency waves complementary to the phase shift of such waves throughsaid coupling means and which are of such magnitude with relation tosaid coupling means as to produce a net voltage response through saidcoupling means and said network for such low frequency wavessubstantially equal to the large response at said high frequencies.

DONALD E. NORGAARD.

